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Air

If the system was not flushed by at least a flow velocity of 2 feet per second, it could still have air in the system. Shut the system down and isolate the loops and pressure check with at least 40 psi of pressure. Do this with water and if you can get more great. The pipe will hold up to 160 psi. If it leaks down, you can see how big the leak is roughly and if it's not that big, they make a additive to help stop leaks. Hope this helps

also when shut down the air should rise to the highest point in the loop, if accessable you can purge it out there. But generaly it is under some pressure on return and definetly on supply it should not suck air, gauging the pressure differance of supply and return will determine if return is in a - pressure.

The company I work sells waterfurnace and we have a jaccuzzi pump with an 8 inch pe tube that has a supply at the bottom and return at the top. You run that thing and watch the bubbles dissapear, also used to add anti-freeze. Ofcourse the loops should be pressure tested with nitrogen or air first.

Is it correct to say that there should NEVER be "air" in a closed loop system?
I have two different homes that are getting air on a regular basis.

True. But amount depends on type of sytem. Some low pressure systems will handle quite a bit and be perfectly happy untill it purges out. Once you find the manifold, run 24 hour leak tests on each loop. The bad one is typically very easy to see.

I am too long in the tooth to retool for loop work. So, I got a loop guy to travel 100+ miles to fix this mess for once and for all!

We replaced all PVC with weld pipe. Then we flushed, flushed and flushed again! This was especially scary in the 3rd floor attic units (the only way I got approval for this work was that I had to be there to oversee it).

Imagine dragging pumps, pipes, fittings and water up 3 floors to the attic in a house filled with antiques. And then pumping "A LOT" of water (the loop pond was 1500+ feet behind the house).

Good work, Happy.
The first loop our company sold 10 years ago we did the same thing. Now that equipment is depreciated, it is lucrative work. Never flushed or filled one in an attic.....yuck! Basements and crawl spaces are bad enough.

loop air

Is it correct to say that there should NEVER be "air" in a closed loop system?
I have two different homes that are getting air on a regular basis.

One has a "professional installed" weld pipe loop to a pond.

One is an engineer type DIY loop system.
They both add water and bleed the air. On the professional loop we are throwing in the towel and starting to dig/find the leak.

We had a job like this and it took forever to figure it out. It turned out to be a bad seal on the pump kit itself that never leaked enough to leave water on the floor, but enough to trip off the water flow sensor. What a relief to not have to try to float up a pond loop and look for a leak. Not to mention I was off the hook since i did all the heat fusion welds